There has been a demand for variable displacement axial piston hydraulic pumps which can deliver increased power, which can operate at typical electric motor speeds such as 1800 rpm, which are quiet and which utilize inlet fluid at atmospheric pressure. One of the main limiting factors as to the speed at which an axial piston pump may be run is the speed at which fluid at the inlet port fills the piston bores during the pumping operation. If the bores are not filled with fluid as they traverse the inlet port, cavitation occurs, power is lost and severe damage to the pump may occur. Traditionally, users have added boost pumps or otherwise acted to pressurize the fluid at the pump inlet in order to increase the filling speed of the pump and thereby increase the speed at which the pump may be operated.
Serious disadvantages occur when a boost pump or other pressurization means is utilized to increase the pressure of fluid at the inlet port. Such pressure boost systems increase the energy requirements of the hydraulic system thereby decreasing the overall efficiency of the system. Boost systems also adversely affect the operating environment of the hydraulic system in that they increase the overall noise level of the system. In many industrial applications, boost systems are not desired because of increased system costs, complexity, maintenance, difficulty of installation and noise.
The instant invention enables a variable displacement, axial piston pump to operate at a reduced noise level while being driven at relatively high electric motor speeds utilizing inlet fluid at atmospheric pressure. It has been found that in order for inlet fluid to enter the piston bores of a piston pump, the fluid must accelerate to the vector sum of the velocity of the pump inlet ports which rotate along a porting circle (tangential velocity) plus the axial velocity into the pump port. The tangential velocity (feet per second) component may be calculated utilizing the formula N (rpm) divided by 60 multiplied by bore circle diameter (ft.) multiplied by pi (3.14159). In this formula the piston bore circle diameter is equal to the diameter of the porting circle. In the axial piston pump of the instant invention the speed which must be attained by incoming pump fluid has been reduced by reducing the tangential velocity component thereof. This has been accomplished by effectively reducing the diameter of the porting circle. Additionally, Applicant's instant invention uniquely provides a velocity boost to incoming pump fluid by utilizing centrifugal force to further increase the rate at which incoming fluid reaches the velocity of the piston circle. Furthermore, the pump of the instant invention has a port plate designated to reduce the fluid shock and attendant noise which occurs as a piston bore moves from an inlet port to an outlet port and from an outlet port to an inlet port.
In the axial piston pump of the instant invention a barrel bearing affixed to the outer surface of the barrel rotatably mounts the barrel in the pump housing. With this design radial loads which necessarily occur in an axial piston pump from the pumping forces are absorbed by the barrel bearing. In contrast to this design, other axial piston pumps utilize a large, stiff shaft, supported at each end by bearings, which extends through the center of the cylinder barrel to provide support. With this design, radial loads and torque loads from driving the barrel are imposed on the shaft. This requires that the shaft have a relatively large diameter. Removing the barrel support from the shaft through the use of a barrel bearing permits the use of a smaller diameter drive shaft which in turn allows the piston circle i.e. the circle which contains the equal spaced piston cylinder bores in the cylinder barrel to be smaller in diameter. Where the piston circle is the same as the porting circle, the reduced piston circle diameter lowers the tangential velocity component required of the incoming fluid and thus permits the pump to fill at a higher rotational speed.
Applicants have reduced the required tangential velocity component of incoming fluid by reducing the effective porting circle diameter through the use of inwardly angled fill ports. The ports are in fluid communication with the piston bores and have a fill end which opens into the working face of the barrel along a fill circle having a smaller diameter than the piston circle. It has been found that because the fill port circle and the piston circle are different diameters an unbalanced force moment is created which tends to tip the barrel. This moment creates a radial force which is taken by the barrel bearing.